1. Field of the Invention
The present invention relates to an optical recording medium in which information can be recorded using laser light and the recorded information can be reproduced using laser light, such as CD-DAs, CD-Rs, CD-RWs, DVD-VIDEOs, DVD-ROMs, DVD-Rs, DVD-RWs, DVD+Rs, DVD+RWs and DVD-RAMs. Particularly, the present invention relates to a high speed optical recording medium in which information can be recorded at a high speed not lower than 12 m/sec.
2. Discussion of the Related Art
Various optical recording media, in which information can be recorded or reproduced with laser light, such as CD-DAs, CD-ROMs, VIDEO-CDs, CD-Rs, CD-RWs, DVD-VIDEOs, DVD-ROMs, DVD-Rs, DVD-RWs, DVD+Rs, DVD+RWs and DVD-RAMs have been commercialized. Needs such as higher density information recording and higher speed information recording exist for these optical recording media to record a large amount of information at a high speed. Recently, a Ag-based reflection layer has been investigated to provide such a high density and high speed recording medium.
Recording media having a Ag-based reflection layer have the following advantages:    (1) the resultant recording media have increased reflectance over a wide wavelength region, resulting in increase of reproduction ability of the recording media;    (2) the amplitude of recorded signals increases because silver has good optical properties, resulting in increase of reproduction ability of the recording media;    (3) in the case of phase change optical recording media, the reflection layer has relatively high cooling speed, resulting in improvement of the overwriting ability of the recording media;    (4) in the case of phase change optical recording media, the reflection layer has relatively high cooling speed, resulting in widening of the recordable speed range of the recording media;    (5) the resultant recording media have improved productivity because the Ag-based reflection layer has high sputtering efficiency; and    (6) the heat stress applied to the recording media can be decreased because the sputtering time can be reduced, resulting in improvement of mechanical properties of the recording media.
When silver is used for a reflection layer, however, the resultant reflection layer has the following drawbacks:    (1) the reflection layer tends to be corroded under high temperature and high humidity conditions;    (2) the reflection layer tends to be easily corroded by sulfur and chlorine;    (3) the reflection layer has poor adhesion to the layer on which the reflection layer is located; and    (4) silver is a noble metal and therefore the reflection layer has a relatively high cost compared to an aluminum reflection layer.
In attempting to prevent silver from being corroded, Unexamined Japanese Patent Application No. (hereinafter referred to as simply JP-A) 57-186244 (i.e., U.S. Pat. No. 4,709,363) discloses a AgCu reflection layer. In addition, AgMg-based, AgOM-based and AgPdCu-based reflection layers have been disclosed in JP-As 7-3363, 9-156224 and 2000-285517. Further, Japanese Patent No. 2749080 discloses a Ag-based reflection layer in which one or more elements such as Ti, V, Fe, Co, Ni, Zn, Zr, Nb, Mo, Rh, Pd, Sn, Sb, Te, Ta, W, Ir, Pt, Pb, Bi and C are included.
When the present inventors prepared a DVD+R disc and a DVD+RW disc using such materials as a reflection layer to evaluate the archival preservation property thereof at 80° C. and 85% RH, the resultant discs have poor preservation reliability because a reproduction error problem suddenly increases after a 300-hour preservation test.
In attempting to prevent a reflection layer from being corroded, methods in which an ultraviolet crosslinking resin layer is formed on a reflection layer have been conventionally proposed. For example, JP-A 2001-222842 discloses a method in which a resin layer having a glass transition temperature not lower than 45° C. is formed on an Al reflection layer to prevent water absorption of the resin layer, resulting in prevention of wrinkling of the resin layer, and thereby corrosion of the Al reflection layer can be prevented. When the present inventors prepared a recording medium in which the resin having a glass transition temperature of 80° C. which is disclosed in JP-A 2001-222842 is formed on a Ag-based reflection layer, the reflection layer of the resultant recording medium was corroded and thereby a reproduction error problem increases.
In addition, it is known that a ZnS.SiO2 (80/20 by mole) layer is formed on and under a reflection layer to prevent corrosion of the reflection layer. In this case, a ZnS.SiO2 layer having optimized thermal expansion coefficient, optical constant and elastic modulus is used.
However, it is known that when a Ag-based reflection layer is formed on the ZnS.SiO2 layer to prepare a high speed recording phase change optical recording medium, a problem in that the reflection layer is corroded due to reaction of Ag with ZnS.SiO2.
In attempting to solve the problem, JP-A 11-238253 discloses a method in which an intermediate layer using a material such as Ta, Ni, Co, Cr, Si, W, V, C, Si, Au, Pd, Ag oxides, Al oxides and Ta oxides is formed to prevent the Ag-based reflection layer from reacting with sulfur included in the protective layer adjacent to the reflection layer. It is also disclosed therein that the thickness of the intermediate layer is preferably 40 nm to utilize the high heat conductivity of the Ag-based reflection layer while preventing corrosion of the reflection layer. In addition, it is disclosed that when the intermediate layer has a thickness of from 10 to 50 nm, the resultant medium has good signal properties, and good preservation reliability at 80° C. and 85% RH.
However, when the present inventors prepared phase change optical recording media having such an intermediate layer having a thickness of from 10 to 50 nm, the signal properties of the resultant recording media largely depended on the thickness of the intermediate layer, and thereby the recording media could not have practical signal properties. In addition, when the recording media were subjected to a heat cycle test 6 times in which a cycle of preservation at 25° C. and 95% RH for 12 hours, followed by preservation at 40° C. and 95% RH for 12 hours was repeated while the recording media were heated and cooled at a heating or cooling speed of 10° C./hour when the temperature was increased or decreased. As a result, a problem in that the Ag-based reflection layer is peeled from the intermediate layer occurred.
Namely, according to the present inventors' investigation, it is found that the reaction of silver in the reflection layer with sulfur in the protective layer can be prevented but the adhesion of the intermediate layer to the Ag-based reflection layer is poor, and thereby adhesion between both the layers deteriorates when the recording media are preserved under high humidity conditions or dew condensation occurs. The reason is considered to be that by forming a chemically inactive intermediate layer to prevent diffusion of elements included in one of the layers to the other layer and to prevent corrosion of the Ag-based reflection layer, deterioration of adhesion of the intermediate layer to the Ag-based reflection layer cannot be prevented particularly under high humidity conditions.
The present inventors disclose in JP-A 2000-331378 that an element such as AlN, SiNx, SiAlN, TiN, BN, TAN, Al2O3, MgO, SiO, TiO2, B2O3, CeO2, CaO, Ta2O5, ZnO, In2O3, SnO2, WC, MoC, TiC and SiC is used in an upper dielectric protective layer adjacent to a reflection layer, wherein the upper dielectric protective layer may have a multi-layered structure, and wherein the upper dielectric protective layers (i.e., the second and third protective layers 4 and 5 in FIG. 1) preferably have a total thickness of from 7 nm to 60 nm, and more preferably from 10 nm to 30 nm.
However, there is no embodiment therein, in which the thickness of the second upper dielectric protective layer (i.e., the third protective layer 5 in FIG. 1) is 9 nm or less (there is only an embodiment therein in which the thickness of the second upper dielectric protective layer is 10 nm). In addition, there is no technical idea therein such that in order to dramatically improve the reliability of a Ag-based reflection layer without widely changing the optical and thermal properties of the recording medium and in order to prevent corrosion of silver in the reflection layer without adversely affecting the function of the first protective layer (i.e., the second protective layer 4 in FIG. 1), a thin surface-modified layer is formed as the second protective layer (i.e., the third protective layer 5 in FIG. 1) contacting the reflection layer.
As mentioned below, when a material including Si in an amount not less than 35 atomic percent is used for the protective layer (i.e., the third protective layer 5 in FIG. 1) contacting the reflection layer, the initial signal properties and the reliability under high humidity conditions of 95% RH of the resultant recording medium deteriorate if the thickness of the protective layer is not less than 10 nm (this can be easily understood from the evaluation results of Comparative Examples 3 to 7 mentioned later). In addition, when a third protective layer including a material other than the materials including Si in an amount not less than 35 atomic percent such as SiO and SiC (the material is considered to be equivalent to the materials including Si in an amount not less than 35 atomic percent in JP-A 2000-331378) and having a thickness not greater than 9 nm is formed, the resultant recording medium cannot produce the same effects as those of the recording medium of the present invention (this can be understood from the evaluation results of Examples 1 to 16 in Table 1).
Namely, JP-A 2000-331378 does not disclose nor suggest that among the materials for use in the upper protective layer disclosed therein, only the materials including Si in an amount not less than 35 atomic percent can produce excellent effects.
Accordingly, although it is disclosed in JP-As 11-238253 and 2000-331378 to use Si or a material including Si for the intermediate layer or dielectric protective layer, such materials are considered to be equivalent to materials which are considered to be comparative materials in the present application. Namely, it is not disclosed nor suggested therein that only the materials including Si in an amount not less than 35 tomic percent can produce excellent effects. In addition, since the thickness of the protective layer is greater than the preferable range (i.e., from 2 nm to 9 nm) in the present invention, the object of the present invention cannot be attained. Namely, it is clear that the technical idea of the present invention is not disclosed therein.
Further, the above-mentioned patent applications have never disclosed preferable sputtering conditions of the intermediate layer or dielectric protective layer including Si, preferable structures and film properties of the resultant layer, which depend on the sputtering conditions, and preferable layer thickness which is determined such that a good combination of the environmental reliability and signal properties can be imparted to the recording medium.
Because of these reasons, a need exists for an optical recording medium which has good preservation reliability even under high temperature/high humidity conditions, good high temperature operation stability, good mechanical properties, and good productivity and which can perform high speed recording and reproduction.